Noise cancellation systems are known, in which an electronic noise signal representing ambient noise is applied to a signal processing circuit, and the resulting processed noise signal is then applied to a speaker, in order to generate a sound signal. In order to achieve noise cancellation, the generated sound should approximate as closely as possible the inverse of the ambient noise, in terms of its amplitude and its phase.
In particular, feedforward noise cancellation systems are known, for use with headphones or earphones, in which one or more microphones mounted on the headphones or earphones detect an ambient noise signal in the region of the wearer's ear. In order to achieve noise cancellation, the generated sound then needs to approximate as closely as possible the inverse of the ambient noise, after that ambient noise has itself been modified by the headphones or earphones. One example of modification by the headphones or earphones is caused by the different acoustic path the noise must take to reach the wearer's ear, travelling around the edge of the headphones or earphones.
Noise cancellation systems are also known that serve to reduce noise on an outgoing signal. For example, in hands-free telephone headsets, noise cancellation may be applied to the outgoing voice signal, such that a called party is better able to distinguish the caller's voice from ambient noise that is picked up by the microphone in the caller's device. Such noise cancellation systems may employ a voice activity detector so that the voice itself is not cancelled from the outgoing signal.